Project Summary: Cardiovascular disease (CVD) is the leading cause of morbidity and mortality worldwide. Better understanding of the pathological mechanisms underlying CVD will improve preventive and therapeutic interventions. One major etiology of CVD is dysregulation of cardiac gene expression by transcription factors (TFs) and microRNAs (miRNAs). miRNAs are small non-coding RNAs involved in gene expression regulation. TFs are usually poor small-molecule drug targets. However, miRNAs can act as unique therapeutic agents because they can target multiple TFs in the same pathological pathway. Most precursor miRNAs (pre-miRNAs) are processed to generate an active driver strand miRNA; the complementary passenger strand is usually degraded. Intriguingly, pre-miR-574 produces two functional strands?miR-574-5p and miR-574-3p. We found that both strands of miR-574 play a synergistic role to protect hearts from cardiac hypertrophy. We showed that miR-574-5p was induced in chronic human heart failure tissues versus normal hearts. We found that miR-574-5p is up-regulated under hypertrophic and ischemic stress and silences the expression of three pro-hypertrophy TFs myocyte enhancer factor?2 factors (Mef2a/2c/2d) and prevents cardiomyocyte (CM) hypertrophy. Transfection of miR-574-5p mimics in mouse cardiomyocytes (CM) protects the cells from isoproterenol (ISO, a ?-adrenergic agonist)-induced hypertrophy and hypoxia-serum starvation-induced cell death. miR-574 knockout mice exhibit an advanced cardiac hypertrophy phenotype associated with increased fibrosis and enlarged CM, compared to wild-type mice after chronic ISO injection. We also showed that the passenger strand miR-574-3p is co-upregulated with miR-574-5p but captured by hypoxia-activated phospho-hnRNP L (heterogeneous nuclear ribonucleoprotein L). miR-574-3p is secreted from CM via exosomes and targets fibroblasts to reduce fibrosis. Our research goal is to address fundamental questions in miRNA biology using miR-574-5p/3p as archetypal molecules in the cardiac system (differential regulation, mechanistic diversity, and functional relationship between driver and passenger strands), and to treat HF by developing miRNA-based therapeutics that target TFs. Our central hypothesis is: In CM under hypertrophic and ischemic stress, differential regulation of the driver strand miR-574-5p and passenger strand miR-574-3p integrate cardiac gene expression and modulate pathological cardiac hypertrophy and remodelling. We will test this hypothesis by pursuing two aims. Aim 1 will test the hypothesis that miR-574-5p regulates the expression of Mef2 TFs and other cardiac genes via miRISC, thereby modulates cardiac hypertrophy and remodelling. Aim 2 will test the hypothesis that in CM ischemia promotes miR-574 transcription via specific TFs and activates capture of miR-574-3p by P-hnRNP L followed by exosome secretion and intercellular communication.